Within the field of electrochemistry, there is a well-known electrolytic cell called a chlor-alkali cell. Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane such as the commercially available NAFION manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g, asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode.
In this cell, an electric current is passed through a concentrated brine (sodium chloride) aqueous solution to produce chlorine gas and caustic soda (sodium hydroxide) by electrolytic dissociation of the sodium chloride in water. An unwanted by-product of this reaction is production of hydrogen gas at the cathode of the chlor-alkali cell. It has been estimated that as much as 25 percent of the electrical energy needed to operate chlor-alkali cells is consumed by the formation of hydrogen gas at the cell cathode.
Among various attempts to reduce this electric power loss have been the development of the so-called oxygen (air) cathodes, which cathodes eliminate the formation of hydrogen at the cathode by instead reducing oxygen to form hydroxyl ions. The reduction of oxygen instead of the formation of the by-product hydrogen requires less electric power. Hence, it can be seen that electric power savings of as much as 25 percent can be achieved in the operation of the chlor-alkali cells by eliminating the formation of hydrogen at the cathode.
Characteristically, the oxygen (air) cathodes contain catalyst particles such as precious metal particles; or they can contain carbon particles, e.g., active carbon, having a high internal porosity, carbon black, graphite, etc.; or they can contain such carbon particles containing precious metal catalyst, e.g., silver, platinum, etc., deposited in and/or on said carbon particles. The presence of the precious metal in conjunction with the carbon enhances the activity of the carbon to form hydroxyl ions from the oxygen supplied at the cathode in the chlor-alkali cell.
One of the problems encountered with the use of active carbon particles is the corrosive nature of the catholyte, caustic soda, which tends to wet (flood) the pores of the active carbon and in essence restrict or lessen its desired activity. Since oxygen has a very low solubility in the electrolyte employed in the chlor-alkali cell, if the electrolyte fills all of the pores of the active carbon layer, the oxygen cathode is no longer capable of functioning to produce its desired results. Therefore, various materials have been employed in conjuction with active carbon in the active layer of the oxygen (air) cathode in an attempt to avoid the electrolytes wetting or filling the pores of the active carbon particles. Polytetrafluoroethylene (PTFE) and other fluorinated polymers have been employed to impart hydrophobicity to the active layer, per se, and/or to a protective or backing sheet which is laminated or otherwise attached to the active layer, or both to the active layer and the protective or backing sheet attached thereto.
It is known that in order to prepare the polytetrafluoroethylene in stable dispersion form wherein the PTFE particles account for about 60 percent solids content of the aqueous dispersion and have particle sizes ranging from about 0.05 to 0.5 microns with an average particle size of approximately 0.2 micron having a preponderant distribution leaning towards the smaller particle sizes, it is necessary to employ a wetting agent in the manufacture of these dispersions. The wetting agent primarily used in manufacturing the PTFE dispersions useful in accordance with this invention is "Triton X100" which is an anhydrous biodegradable liquid having 100 percent activity as a nonionic surface active agent. This material is water-soluble and is comprised of isooctyl phenoxy polyethoxy ethanol containing 10 moles of ethylene oxide. It is known in the art that this material is soluble in chloroform.
Some efforts have been made to remove the wetting agent by heat treating in air at elevated temperatures, e.g., 275.degree. C. and above. However, such procedures often result in unwanted residue. Heat treating in inert atmosphere at such elevated temperatures is unavailing for removal of such wetting agents. Moreover, heat treating in air can cause combustion (burning) of the active carbon which is undesirable.
In the operation of chlor-alkali cells, e.g., using oxygen (air) cathodes having carbon and PTFE present therein, there has been a trend to increase the current density at which the chlor-alkali cell operates. The purpose of increasing current density is to increase the productivity of the cell, thus reducing the number of cells required to produce a given amount of chlorine and caustic. This substantially reduces the capital cost of a chlor-alkali plant. Moreover, the use of such high current densities, i.e., 300 plus milliamps per cm.sup.2, is increasing in battery applications, e.g., in aluminum air batteries.